1. Field of the Invention
The present invention relates to an apparatus for recovering a symbol timing of a signal received in a receiver of a digital communication system. The present invention is based on Korean Application No. 96-3484, which is incorporated herein by reference.
2. Description of the Prior Art
A transmitter of a general digital communication apparatus modulates a signal by a quadrature amplitude modulation (QAM) method or a quadrature phase shift keying (QPSK) method and converts the modulated signal into an analog signal to transmit it. A receiver thereof demodulates the modulated signal transmitted from the transmitter and converts the demodulated signal into a digital signal to restore it to the original signal. Such a receiver can accurately restore the received signals into original symbol values when the received signals are sampled at the same interval of time and phase as those of the symbols transmitted from the transmitter. Therefore, one of the important signal processings which should be done in the receiver of the digital communication apparatus is to accurately restore a symbol timing representing a point in time to be sampled from the received signal. When a particular carrier corresponding to a sampling frequency is not transmitted, it is very important to accurately recover a symbol timing using only the received signal. A conventional apparatus for recovering a symbol timing is shown in FIG. 1.
In FIG. 1, a received signal is input to an analog-to-digital (A/D) converter 11. The A/D converter 11 samples the input signal at a frequency supplied from a voltage controlled oscillator (VCO) 15 and converts the analog signal into a digital signal. The digitized signal is output via a matching filter 12, and simultaneously input to a timing error detector 13. The timing error detector 13 detects a symbol timing error from the input signal and outputs a detected error value, to a loop filter 14. The loop filter 14 filters the input signal to remove various noises. The VCO 15 oscillates at a frequency according to the output of the loop filter 14 and supplies the oscillated frequency signal to the A/D converter 11.
Since analog components are used in the hardware implemented circuit, the apparatus of FIG. 1 is greatly affected by temperature and environment. Due to such effects, the apparatus does not appropriately trace an interval of time of symbols which are transmitted from the transmitter. As a result, a symbol timing drift phenomenon occurs in which the number of symbols transmitted from the transmitter differs from that of restored symbols.
To solve this problem, a full digital implementation of a symbol timing recovery apparatus, as shown in FIG. 2, was developed. In FIG. 2, a fixed oscillator 22 generates a constant frequency signal and supplies the same to an A/D converter 21. The A/D converter 21 converts the received signal into a digital signal using the frequency signal supplied from the fixed oscillator 22, and outputs the digitized signal to an interpolator 23. Meanwhile, a controller 27 receives a symbol timing error signal output a timing error detector 25 via a loop filter 26. The controller 27 generates an interpolation coefficient according to the input symbol timing error signal and supplies the same to the interpolator 23. The interpolator 23 interpolates the digitized signal with the interpolation coefficient supplied from the controller 27, and outputs the interpolated result to a matching filter 24.
The A/D converter 21 samples the received signal at a sampling frequency supplied from the fixed oscillator 22 and converts the analog signal to the digital signal. The digitized signal is output via the interpolator 23 and the matching filter 24 and simultaneously input to the timing error detector 25. The symbol timing error signal which is detected in the timing error detector 25 and has noise removed in the loop filter 26 is then input to the controller 27. The controller 27 generates the interpolation coefficient according to the input signal. The interpolator 23 interpolates the digitized signal according to the interpolation coefficient supplied from the controller 27, and thereby removes the symbol timing error. Although this apparatus can accurately recover the symbol timing, it is very complicated to implement using hardware.